Homebuilt ROVs Forum

Have finished an ROV in a box and used it twice in testing. Once in a pool for several hours and later in the bay. The vertical thruster failed at home while rinsing off. After some troubleshooting I opened the unit up and discovered moister and rust inside. This is a brand new unit so am surprised it failed and also that it somehow allowed water inside.

The thruster is a Johnson 2855 500 gpm 12 VDC motor. The only way water can get in as far as I can tell is through the rubber shaft seal.

Has anyone else had a thruster fail like this? I'm wondering if I can trust the other two motors?

Hi Sailman,

I had been inactive in the forum for a while, so I just saw your message. Although after 3 months from your post you'll have most probably found a solution,
I decided to share some experience on bildge pumps.
I also had an ROV-in-a-box kit with the same 500gph Johnson bildge pumps as you have. Two out of three motors, also failed after about 20 dives with the exact same symptoms as in your case. Totally rusted inside.
I had worked them in the sea and sent the ROV down to approx. 20m repeatedly for at least 5 dives. When the mission was over, everything worked fine but after one month of storage in the garage
the two motors were out of order.
Yet, in all bildge pumps there are 2 ways for the water to come in. One way through the shaft seal and the other through the wire's seal. When I opened the motors I could not say which one had failed.
Anyway, in my new motors (Johnson L1600gph) I put some epoxy on the wires at the back end of the motor, just to add some more protection.

Hope it helps,

Triton

I have never been convinced that bilge pump motors are really all that watertight.
They are just enclosed so you cant tell that they have water in them.
I know of some that had water in them after only going as deep as the bottom of a pool.
I would rather run an open motor so the water can drain out...
But that's just me.

Thanks for your comments Triton. I agree with the two possible water entry points but I'm guessing the wiring port is probably a pretty good seal. The shaft bearing on the other hand is only a rubber hole as best I can tell. Anything done that changes the diameter of the hole will allow a leak.

I had a Chrysler outboard many years ago that would eat up a water pump impeller in seconds if it were run without water for lubrication. Could the bilge pump motor also develop a leak if it were run without water? Could the rubber heat up and enlarge the hole?

Also I noted that bilge pumps are operated with the shaft down. The case acts like an empty glass places upside down in a bowl of water. No leaks there. I wonder if perhaps these motors were not intended to be run in any position other then with the shaft down?

As to running the motors wet scubasteve...the metal parts on these Johnson units are highly susceptible to corrosion. I don't think they would last more than one dive if they were open. What type motors do you use when they are exposed to water? My comments aren't meant to be sarcastic so please don't take offense.

No offense taken. My comments may just be the ramblings of an idiot.
Anyway, I'm using brushless motors on my larger rov and brushed on a little one for shallow water use.
I expect I'll have to spray them down with something between dives to prevent corrosion but I think that's preferable to it holding water when in storage.